Randi Zuckerberg, sister of Facebook billionaire Mark, has expressed her dismay over how a private photo she posted on the social network made it to Twitter without her permission.

The 30-year-old, who works as Facebook's marketing director, posted a photo to her Facebook page showing her family's reaction to the new "poke" application, a photo-sharing tool that quickly soared to the top of the charts for iPhone users. A friend-of-a-friend, who saw the picture, reposted it on Twitter, causing Randi to see red.

"Not sure where you got this photo. I posted it to friends only on FB. You reposting it to Twitter is way uncool," she said, according to Buzzfeed.

Callie Schweitzer, director of marketing and projects at Vox Media, responded that as a subscriber to her Facebook feeds, she saw the picture and thought nothing of reposting it on the rival social network.

Facebook has long caused concern for its users over its privacy settings, which are routinely overhauled, as many times, photos and messages previously thought private, have gone public.

Facebook and Twitter users who saw Randi's confusion over how her photo ended on Twitter were quick to point the finger at her company's own privacy settings which continuously catch people out. However, Randi, who deleted her earlier post, clarified that it wasn't about Facebook but her sensitivity to private photos becoming "news."

"Digital etiquette: always ask permission before posting a friend's photo publicly. It's not about privacy settings, it's about human decency," said Randi in an updated post on Twitter.

However, considering that she had failed to correctly tag the photo as for "friends only," perhaps the problem really is with Facebook's privacy settings.